- Today, it works.
- Tomorrow, it crashes.
- Next week - where the !$?" are my OS install disks!?

Not a lot can happen in a user land process; the OS will kill off any process trying to access illegal memory.

But if you were programming a kernel mode driver, or writing code for a bare board, then all bets are off. Pretty much anything can happen, including causing physical damage to hardware (the kind that involves spending money to replace).